The present invention relates to a fuel injector.
In German Published Patent Application No. 35 35 438 is discussed an electromagnetically operable fuel injector which has, in a housing, a solenoid coil surrounding a ferromagnetic core. A flat armature is arranged between a valve-seat support permanently joined to the housing, and the end face of the housing. The flat armature cooperates with the housing and core via two air gap insurances (or working air gaps), and is guided radially by a guidance membrane which is mounted to the housing and embraces a valve-closure member. The connection between the flat armature and the valve-closure member is produced via a ring that surrounds the valve-closure member and is welded to the flat armature. A helical spring applies closing pressure to the valve-closure member. Fuel channels, as well as the geometry of the flat armature, particularly the depression of the regions adjacent to the fuel channels, allow fuel to circumflow the armature.
It is believed that a disadvantage of such a fuel injector is the high cavitation tendency through the large cavities, traversed by the fuel, in which fluxes and swirl effects develop. Because of the high resistance to flow, the displacement of the fuel upon pull-up of the armature may take place in a delayed manner, and therefore may have disadvantageous effects on the opening time of the fuel injector. In addition, the cavitation is intensified due to the position of the flow-through openings which are placed not at the apex, but rather in the flank of the flat armature.
In German Published Patent Application No. 31 43 849, a similarly formed flat armature is used in a fuel injector. It may be that in this case, the flow-through openings are placed at the apexes of the flat armature; however, due to the armature edge which is still raised, is aligned parallel to the armature stop face and makes displacement of the fuel into the edge areas of the armature impossible, it is believed that the hydrodynamic properties are not essentially improved.
In European Patent No. 0 683 862 is discussed an electromagnetically operable fuel injector whose armature is characterized in that the armature stop face facing the internal pole is slightly wedge-shaped in order to minimize or completely eliminate the hydraulic damping upon opening the fuel injector and the hydraulic adhesion force after switching off the current energizing the solenoid coil. In addition, owing to suitable measures such as vapor deposition and nitration, the stop face of the armature is wear-resistant, so that the stop face has the same size during the entire service life of the fuel injector, and the functioning method of the fuel injector is not impaired.
Disadvantageous in such a fuel injector is that, in spite of the optimized armature stop face, primarily the hydraulic damping force still exists in the working gap upon pull-up of the armature. If an excitation current is applied to the solenoid coil, the armature moves in the direction of the internal pole and, in so doing, displaces the fuel present between the internal pole and the armature. Because of frictional and inertia effects, a local pressure field builds up which produces on the armature stop face a hydraulic force that acts counter to the moving direction of the armature. The opening and fuel-metering times of the fuel injector are thereby prolonged.
The exemplary fuel injector of the present invention is believed to have the advantage that, by suitable geometric design of the armature, the hydraulic damping force is considerably reduced and thus the fuel injector can be opened more quickly, resulting in more precise metering times and quantities.
A favorable geometry of the armature stop face is achieved by the opposing slope of the edge areas of the armature stop face. The armature possesses two annular edge zones, the inner edge zone being inclined inwardly toward the inside radius, while the outer of the edge zones is inclined outwardly toward the outside radius. The armature stop face is therefore bounded by sloped surfaces. The slope angle of the boundary surfaces influences the flow behavior of the fuel in the working gap. The armature stop face is reduced in size by the geometric design, which means the area subject to wear is smaller.
It is also believed that an advantage is provided by the placement of axial channels in the armature which provide the fuel present in the working gap the possibility of flowing off through them upon actuation of the armature. The channels are arranged in depressions, the flow behavior thereby further improving, since the fuel can escape without delay through the armature.
The same effect can also be attained by cutouts which are spaced evenly at the outer edge of the armature. In this case, due to the outwardly beveled shape of the armature stop face, the fuel is displaced to the outer edge of a central fuel-injector opening accommodating the armature and can flow off through the cutouts in the armature.
The depressions can be bounded by one sloping and one perpendicular surface. Another exemplary embodiment provides for a different height for the raised annular apexes formed by the inclined surfaces, so that only a minimal surface is used as the armature stop face.
An annular cutout at the magnetic surface in the region of the solenoid coil brings about a positive influence on the hydraulic damping due to a local enlargement of the working gap.